1. Field of the Invention
The present invention relates to a system for detecting an underground object such as a cable or pipe, using a detector mounted on a construction vehicle such as an excavator vehicle.
2. Summary of the Prior Art
The proliferation of networks of buried cables and pipes from many different utilities (electricity, gas, telecommunications etc) has meant that an excavation of the ground is likely to be in the vicinity of a buried cable or pipe, and such excavation involves a risk of damage or interference to the buried cable or pipe. This problem is particularly acute when the excavation is carried out by a powered excavating tool such as a back hoe mounted on an excavator vehicle. Because of the separation of the operator of the excavator vehicle from the point of excavation, it is very easy for the excavating tool to make contact with the buried cable or pipe, particularly when the exact location of the buried cable or pipe has not been accurately established and marked in a way which is readily visible to the operator of the excavator vehicle. Moreover, the power of excavating tools is large, so that any contact between the excavating tool and the buried cable or pipe is likely to result in damage to that buried cable or pipe.
It is known to mount a detector on the excavating tool itself, to detect currents in the underground conductor and to generate an alarm or to prevent movement of the excavating tool in a way that would cause the excavating tool to approach too closely to the underground conductor.
The present invention, on the other hand, proposes that sensors be mounted at different heights on the excavator(construction)vehicle itself, with each sensor having at least one magnetic field detector for detecting a component of magnetic field which component is generally parallel to the ground on which the construction vehicle stands or moves. The sensors detect the magnetic fields generated by currents in the underground conductor.
In such an arrangement, when the excavator vehicle is relatively remote from the underground conductor, the or each upper sensor detects a larger signal than the or each lower sensor, because of the difference in angle of alignment of the field, and thus the difference in the components parallel to the ground. However, as the vehicle approaches the underground conductor, that effect decreases, but the effect of difference in separation between the underground conductor and each sensor increases. Once the sensors are directly above the underground conductor, the lower sensor detects a larger field.
Thus, there is a zone on either side of the underground conductor in which the lower sensor detects a larger field, with the upper sensor detecting a larger field beyond that region. The cross-over points between these two regions may thus be used to trigger an alarm. This means that a detection zone is formed between the cross-over points. The operator of the vehicle may be presented with an alarm when the vehicle such that an underground conductor enters into that detection zone. Most excavator vehicles drag the excavating tool behind them, the excavator tool can be prevented from coming into proximity with the underground conductor. It should be noted that this effect of upper and lower sensors is known per se from U.S. Pat. No. 4,427,942. The exact positioning of the sensors on the vehicle is not critical, and indeed it is possible for them to have a small displacement in the horizontal direction, relative to each other.
As mentioned above, the construction vehicle incorporating the present invention is preferably an excavator vehicle, since the present invention finds particular applicability in preventing the excavator tool of such a vehicle of coming in to proximity with an underground conductor. However, the present invention may be applicable to other types of construction vehicle, such as a heavy vehicle which could cause damage to the conductor if it were to drive over it. Moreover, since the present invention creates a detection zone, it is also possible to operate the vehicle so that it remains in the detection zone, i.e. it permits the vehicle to follow the path of the conductor and to generate a warning to the operator if the vehicle departs from the path of conductor, and the detection zone no longer encompasses the conductor.
The sensors may provide outputs to a processing apparatus which is frequency selective, so that it is possible to detect the presence of conductors carrying mains power, and/or conductors to which a signal has been supplied at one or more known frequencies.
The magnetic field detector or detectors of each sensor may each be in the form of a single coil with a axis which is generally parallel to the ground on which the vehicle stands or moves, the axes of the coils being generally parallel. However is also possible for each upper and lower sensor to have orthogonal, horizontal, pairs of coils, so that the horizontal orientation of the conductor relative to the sensors (assuming the ground is generally horizontal) does not affect measurements. A further possibility is for the magnetic field detector or detectors to be in the form of magnetometers, which again may be provided as a single magnetometer for each sensor, or plural magnetometers. Furthermore, since the aim of the sensors is to detect the component of the magnetic field which is parallel to the ground, arrangements of coils and magnetometers may be provided in which the coils and magnetometers themselves are not parallel to the ground, but which are arranged to detect the appropriate component of the field. For example, two coils inclined in opposite directions at approximately 45 degrees to the plane parallel to the ground can be used to determine the component parallel to the ground by algebraic processing of their outputs.
Furthermore, although the present invention has been described as applicable primarily to a vehicle moving on land, this again is not essential and the vehicle may be e.g. a submarine following an under water cable, or a cable buried in the floor of the sea.